Problem with Two Stroke Build

[Schnizz]

Hi folks, first post here so some background first. I recently built Dave Hunter's Two Stroke design from scratch (not the kit). The purpose is two-fold: one is to have a nice amp to play with but also it is to be something of a research project to learn what makes tube amps tick. I have a background in EE but I'm at the just-enough-to-be-dangerous stage when it comes to tube amplification.

The problem I'm having is that when I plug my guitar in, I get a bad scratchy noise when I adjust the volume pot on the guitar. Also, just plugging in the instrument cable causes a lot of noise. At first, I suspected the guitar but it doesn't do that with any other amp. From doing some searches here, I gather that this is caused by DC leakage into the guitar from the amp. Is this correct? What I haven't yet uncovered in searching is what causes that leakage and how to eliminate it.

Another (related?) problem is that I don't get a lot of volume from the amp until the guitar volume is turned all the way up. When I do dime the guitar, the tone from the amp is really good but anything less than full tilt boogie results in a muffled tone.

If this is a common problem that has been solved over and over on this forum, please point me to something I can search for to uncover the answer. If not, any help at all towards finding a solution would be greatly appreciated. Thanks in advance for your time and assistance.

[Schnizz]

Turns out that I had wired up the input jack backwards. So the bottom end of the cathode resistor was floating. I've rewired it and I think it's correct but I still have a problem. Somewhere there's a short from the preamp input to ground so naturally, the amp isn't making a peep other than the slight hum from the filament wires. I'll have to start lifting leads and track down where that ground connection is coming from.

[Enzo]

Maybe you could post a schematic.

[Schnizz]

I'll post one shortly, Enzo. I assumed that the two stroke was fairly well-known. It's basically a 5F2 with a couple of mods, one being a boost switch that switches in either a .47uF or 25uF cathode bypass cap for the 1st preamp stage. The other is that there are two output tube sockets wired in parallel so that you can run the amp with a 6L6, a pair of 6V6s, an EL34, etc.

I created a schematic in Visio so let me convert it to JPEG and put it online.

Edit: Ok, here's the schematic.

[Enzo]

If the amp makes no sound, lift that 68k input resistor from the jack, and touch the end of it with your finger. Does that make loud hum? Should. If not, there is trouble in the amp. If it does, you stil have jack wiring trouble.

Are you using the classic Switchcraft jack for the input? If so, look carefully at its construction. Each element of the jack has its own metal piece in the lakers of the main body. Note which lug serves which element. MAke SURE each lig is wired correctly to the amp.

Scratchy pot comes from DC on the pot, and connecting directly to that grid allows the grid leak voltage to appear on your volume control. Of course the tube could also be faulty, try another.

Also, I am betting that boost switch pops when you switch it. To cure that, get a couple 1 meg resistors. Wire them to the grounded end of the switch, then their other ends go to the to cap legs coming into the switch. In other words the resistors are across the two sides of the switch.


The resistors allow the caps to remain charged, but they are large enough they don;t allow the caps to have any effect on the sound. Then the switch shorts across the resistor, thus enabling the cap.

[Schnizz]

If the amp makes no sound, lift that 68k input resistor from the jack, and touch the end of it with your finger. Does that make loud hum? Should. If not, there is trouble in the amp. If it does, you stil have jack wiring trouble.

Will do, Enzo. I haven't had time to mess with it in the last few days but I will start here. I'm pretty sure that the trouble is either in the jack or the shielded wire to the input of the preamp. The amp was making good tone before I rewired the jack (other than the scratchy pot) so I feel like the rest of the amp is ok.

Are you using the classic Switchcraft jack for the input? If so, look carefully at its construction. Each element of the jack has its own metal piece in the lakers of the main body. Note which lug serves which element. MAke SURE each lig is wired correctly to the amp.

I'm not sure who makes the jack. I'll check but it well could be as you describe. I will double check the jack wiring very carefully. I believe this is where the problem most likely lies.

Scratchy pot comes from DC on the pot, and connecting directly to that grid allows the grid leak voltage to appear on your volume control. Of course the tube could also be faulty, try another.

I've swapped tubes with a known good one so I don't think that's it. The jack was mis-wired before and I'm sure that was the source of the scratchy pot. It's most likely still mis-wired but in a different way.

Also, I am betting that boost switch pops when you switch it. To cure that, get a couple 1 meg resistors. Wire them to the grounded end of the switch, then their other ends go to the to cap legs coming into the switch. In other words the resistors are across the two sides of the switch.

The resistors allow the caps to remain charged, but they are large enough they don;t allow the caps to have any effect on the sound. Then the switch shorts across the resistor, thus enabling the cap.

Yep, I've already figured this much out from reading some posts on here. I happen to have a couple of spare 1 megs that I'll wire as you specified.

Thanks again for the help. I'll probably have a chance to look at it tomorrow and I'll let you know what I find. You're a big help; much appreciated.

[Schnizz]

Success! The problem actually wasn't in the jack at all. I used a shielded coax cable to go from the 68k grid resistor to the preamp input. The shield was grounded to a ring lug on one of the socket's mounting screws. At some point, I must have gotten it too hot and melted the insulation between the shield and the center conductor. This allowed the shield and conductor to make contact at least intermittently. I just replaced that shielded cable and now the amp is playing beautifully.

By the way, I'd endorse this project for any first time amp builders out there. It's easy enough to not be too daunting but certainly not trivial (as I think I've demonstrated). It sounds really good too. Has a very pure clear tone and breaks up nicely at high volumes with some touch sensitivity. Seems to be a lot of fun to play.

Enzo, many thanks for your help.

[Enzo]

And there is a lesson we all learn over and over: the wires themselves. Wires tend to get invisible on schematics when we are looking for a problem - we take them for granted. it never occurred to me to ask if you had used shielded lead wire, but it is certainly not unusual for the input wire.

[Schnizz]

Absolutely. I got to thinking about the problem yesterday at work when a co-worker came in to my office to ask me if I had found the problem. He's a musician and an engineer too so he's been following my progress. I told him that I hadn't had a chance yet to look at it but was planning to after work. He asked what I thought the problem was. So I whipped out a hardcopy of the schematic and said that I was leaning toward something in the input jack since that was the only thing I had changed. But as I looked at the schematic, something about that didn't sit right with me. If it was the jack, I should have been seeing 68k to ground at the preamp grid rather than a dead short. That's when I first started to suspect the lead wire.

So when I got home, the first thing I did (after discharging the filter caps ) was lift the input jack off the chassis. The short was still there. Now I know it's one of three things: one end or the other of the lead wire or the wire itself. I lifted the connection to the grid and measured from the conductor to the shield and it was shorted. Lifted the other end and it's still shorted. So then I knew it was the wire.

If you'd like to see some photos, I'll post some in the next couple of days.

[PeteG45]

Also, I am betting that boost switch pops when you switch it. To cure that, get a couple 1 meg resistors. Wire them to the grounded end of the switch, then their other ends go to the to cap legs coming into the switch. In other words the resistors are across the two sides of the switch.


The resistors allow the caps to remain charged, but they are large enough they don;t allow the caps to have any effect on the sound. Then the switch shorts across the resistor, thus enabling the cap.

Sorry to resurrect an older post but I have a question.

What would this look like? If the switch lugs were numbered like this:

1--2 < .47uF 50v bias cap
3--4
5--6 < 25uF 50v bias cap

and 3 would be the connection to ground

Would the resistors go from lug 3 to lug 2 and from lug 3 to lug 6?

[Schnizz]

In my case, it's an SPDT switch so there are only three lugs. If you go to the schematic, it's the switch labeled "BOOST" underneath the input stage. The resistors that Enzo recommended are not shown in the schematic, but they are connected from the center terminal to each of the other two. Also, you can see the switch in this photo. It's the blue one in the upper right corner of the chassis. It's a bit hard to see due to the angle.

If you were using a DPDT switch, you'd only need to use one of the poles. I *think* that the resistors would go from 2 to 4 and 4 to 6 but I may have misunderstood how you're wiring the caps. Hope that helps but if you still have questions, please feel free to ask.

[PeteG45]

Thanks Schnizz that clears it up nicely. The extra pole may make it easier for me as it gives me a little more room to fit the extra resistors in place.

[Bruce / Mission Amps]

I'd consider (read that as experiment) moving the NFB lead from the switched part of the impedance selector, directly to the 8 ohm tap of the OT and just leave it there... even though there might not be an 8 ohm speaker connected all the time.

[PeteG45]

Bruce, that sounds simple enough and easy enough to change
if the experiment doesn't work out as intended. This all started
from a 5F2A circuit and then I started making changes. Looking
at what I have now it might have been easier to start with the
the Two Stroke. Either way this is where I've ended up.

I do have one question regarding the cathode bypass caps on
the 12AX7. Is the voltage rating of 100v for the 0.47uF cap
adequate? The 25uF is 50v so I figured that it would probably
be OK but I figured that I better ask. The Two Stroke plan
indicates a 400v cap.

[bclick]

Sorry to post in this old thread, but I noticed that someone had put 2 rectifier tubes in the schematic at http://www.math.fsu.edu/~gmizell/pho...0Schematic.png.

Is this just an experiment to handle large power tubes or a recommended mod?

I'm going to build this amp with 2 power tubes, but don't need two 6L6s - 6V6 is enough.

Thanks!